1. Field of the Invention
The present invention generally relates to a method and apparatus for reducing particulate contamination of semiconductor wafers during plasma processing an, more particularly, to removing particulates in a plasma tool through steady state flows and to a system for removing particulates in a plasma using a multipole magnetic field. The magnetic field produces a plasma flow and continues the plasma flow through any stagnation point, towards an opening in the magnetic field, to drag entrained particulates through the opening for removal from the system. With the invention, particulates are reduced in a plasma (and thus the object being processed) by generation of the steady plasma flow.
2. Description of the Related Art
Semiconductor wafers, typically made of silicon (Si), are subjected to several processes during the manufacture of integrated circuits (ICs). Some of these processes involve a plasma (e.g., used in etching, plasma deposition and sputtering). Radio frequency (RF) and direct charge (DC) Glow discharge plasmas, for example, are extensively utilized in the manufacture of ICs. Both electropositive and electronegative gases are used to produce plasmas. Electronegative gases (i.e., those having a higher ion density than an electron density) such as CF.sub.4, CHF.sub.3, Cl.sub.2, HBr or O.sub.2, present difficult contamination problems for semiconductor manufacturers.
Contaminants (e.g., contaminating particulates) ranging in size from less than tenths of microns to several microns are produced or grown in the plasmas or "pulled in" (e.g., assimilated) from adjacent surfaces of a processing chamber or the object itself. The particulates normally have a negative charge, and are produced, for example, by negative ions which are trapped in the plasma by plasma sheaths. Plasma sheaths form where the plasma comes in contact with a solid and act as a boundary layer between the plasma and the solid surface. The size of the sheath is inversely related to plasma density.
Increasing densities of semiconductor circuits make contamination a serious problem to product reliability and may even be a barrier to achieving theoretically possible higher density circuits. Contamination by particulates also is a severe problem in the processing of semiconductor devices such as semiconductor wafers or the like. Indeed, for proper fabrication and processing of the semiconductor devices, the devices must be reliably cleaned of particulates to a size of approximately one-tenth (0.1) of a micron.
Particulate contamination is a major problem encountered during plasma processing of microelectronic materials. By some accounts, 50% of current semiconductor chip yield loss may be attributed to direct or indirect effects of particulate contamination during fabrication. This fraction is expected to increase as device dimensions are reduced in future technologies. Particles that reduce process yields today range in size from the macroscopic to the sub-micron size.
Particulate contamination also has an extremely deleterious effect on the performance and reliability of microelectronic devices produced by plasma etching or deposition. Particulate contamination can result in device failure, poor film quality, changes in material resistivity, and impurity permeation. Further, as device dimensions are reduced, tighter control of the etching profile requires ever more stringent restrictions on the allowable particle contamination number, density, and size. To meet these requirements, tightly controlled, clean rooms are required to avoid particle deposition on product surfaces during wafer transport and handling.
Improvements in clean room technology and in the handling of in-process substrates (for semiconductors and other applications) have reduced the once appreciable introduction of particulates onto substrates during non-process exposure such as wafer handling and transfer. Particulate formation during process steps, including plasma processing, may now contribute a significant fraction of total contamination exposure with corresponding yield reduction.
Additionally, the industry trend is towards integrated vacuum processing", or "multi-channel processing". Thus, surface contamination previously removed by wet or dry mechanical means will be more complex or impossible to remedy since it now requires removal of the substrate from the vacuum chamber. In multi-chamber tools, particulates which drop onto a wafer before, during, or at the completion of a process step may have an especially severe impact on subsequent process steps in that tool.
Recent studies have shown that certain etching plasmas can produce particulates which may be a significant source of product contamination and device failure. These experiments have shown that particles can be nucleated, grown, and suspended in a process plasma until they are significant in size. For example, particles are formed with sizes on the order of the submicron scale to hundreds of microns in diameter. The particles may ultimately fall onto devices being fabricated in the same manufacturing environment. If particles fall before or during film deposition or pattern transfer, then they can disrupt the process step. If the particulates fall at the end of a process step, the particulates may disrupt subsequent process steps. These contaminants often produce defects which affect the device yield, performance and reliability. Similar results have been observed in deposition type plasmas (e.g., PECVD Silane).
The effects of particulate contamination can be magnified when selective plasma etching processes are used. Certain plasma etching processes rely on a combination of feed gases and etching conditions to etch material surfaces on the wafer selectively. The chemical deformation of particulates which are etched at a slow rate in these highly selective plasmas results in micromasking, or an irregular surface often referred to as "grass". This spike or hill of unetched material will also degrade the device performance and reduce process yield.
The presence of these particulates is not always due to material flaking from chamber walls, but may also be due to gas phase processes such as homogeneous nucleation. This suggests that particle contamination problems may not be eliminated solely by rigorous attention to clean room techniques and frequent cleaning of manufacturing equipment. Instead, since the plasma itself can result in product contamination, this problem may pose a "base level" of contamination even with the highest clean room technology.
In a normal plasma, sheaths are formed with electric fields on the order of 100 V/cm. These Sheaths form to reduce diffusion of very mobile electrons which have energies 5 to 10 times greater than that of the heavier ions in the plasma. Since the field in the sheaths retard the negative electrons, the low energy negatively charged particles are trapped in the plasma. As these particulates reach micron size, the ion drag forces push these particulates toward the plasma sheaths were they remain under normal plasma conditions. When plasma undergoes transitions in density caused, for example, by variations in the generator power, microarcs, or the plasma being turned off, these particulates can fall or are pulled to the wafer surface and adhere thereto.
In plasmas with magnetic surface confinement, i.e., multipole or cusp plasmas, the plasma is also bounded by the magnetic fields. In these plasmas shown in FIG. 1(b), the plasma is bounded by the magnetic field to a region which is slightly larger than the wafer being processed. The plasma is bounded by sheaths which prevent the particulates from leaving the plasma except during perturbations of the plasma density as previously described.
Further, it has been found that small variations in the surface adjacent to the plasma causes "particle traps" such as a small trench and a step. These surface variations can have a size as small as a few millimeters and particulates will be trapped in the sheath above these surface variations. Even the edge of the wafer being processed can cause particles to be trapped at the wafer's edge. Additionally, if the plasma density is slightly reduced above the center of the wafer, particles will be trapped above the wafer, thereby falling or being pulled toward the wafer when the plasma is turned off.
In view of the foregoing, it is therefore necessary to provide a way to reduce particle contamination on semiconductor wafers during plasma processing with a plasma tool.